This invention relates to a method and an apparatus for recording and reproducing a digital signal using a rotary head. Particularly, this invention relates to a rotary head type digital signal recording and reproducing apparatus having a recording function to record subcodes for retrieving program number an Automatic Track Finding (which will be abbreviated as ATF hereinafter) signals in conformity with the DAT standard (Industry Standard for the Digital Audio Tape System) on a portion of tape track a fixed time period from the initial end and a portion of tape track a fixed time period immediately before the terminating end of each recording track formed by a rotary head on a magnetic tape, and to record digital audio signals on the track intermediate portion except for the above portions, and a reproducing function to conduct an after-recording operation using one of the above ATF signals at the time after recording (over-write) of the above subcodes, and to carry out a high speed and excellent reproduction of previously recorded signals on the magnetic tape thus recorded.
The background art will be explained with reference to the drawings. In a DAT (Digital Audio Tape Recorder) using a rotary head among those DATs capable of recording pulse code modulated sound data obtained by applying pulse code modulation (PCM) to an analog audio signal on a magnetic tape and reproducing them therefrom, as shown in FIG. 11, the magnetization pattern to be recorded on the magnetic tape is recorded with the azimuth angles of adjacent tracks being different from each other and with guard bands between tracks being absent. In such a DAT, subcodes and ATF signals are recorded on subcode areas and ATF signal areas which are located at a portion of tape a fixed period from the initial end of each track and a portion of tape a fixed period immediately before the terminal end thereof, respectively, and PCM sound data are recorded on PCM sound areas of the track intermediate portion except for the above areas in accordance with a predetermined signal format. The data thus recorded are reproduced.
FIG. 7 is a schematic view for explaining the tracking operation when the PCM sound data recorded on the tracks on a magnetic tape is reproduced.
In this figure, B, A and B represent a track, T.sub.p a track pitch, f1 a pilot signal, f2 and f3 synchronizing signals of an ATF signal, and SP1 and SP2 each represent a respective sampling pulse. The frequency of the pilot signal f1 is a low frequency where little azimuth effect occurs. The ATF signals f2 and f3 have two kinds of signal lengths, respectively. The ratios of these signal lengths are "0.5" and "1".
The signal length of the synchronizing signal f3 on the track B of the first step is "0.5" and the signal length of the synchronizing signal f3 on the track B of the third step is "1". According as the head runs on the track A of the second step, the pattern of the synchronizing signal f3 is such that different signal lengths of "0.5" and "1" appear in turn. The signal length of the synchronizing signal f2 on the track A of the second step is "0.5" and the signal length of the synchronizing signal f2 on the track of the fourth step not shown is "1". Thus, accordingly as the head runs on the track B of the first step or the third step, the pattern of the synchronizing signal f2 is such that different signal lengths of "0.5" and "1" appear in turn.
At the time of running of the head, a head having a width 1.5 times larger than the track pitch T.sub.p running on the track of the second step reproduces the synchronizing signals f2 while reproducing crosstalk components of the pilot signal f1 on the track B of the first step. Further, at the time when the sampling pulse SP1 is output, this head carries out sampling of the crosstalk components of the pilot signal f1. After this sampling, immediately after the head has reproduced crosstalk components of the pilot signal f1 on the track B of the first step, it begins reproducing crosstalk components of the pilot signal f1 on the track B of the third step. At the time when a sampling pulse SP2 occurs after a predetermined time from the time when the sampling pulse SP1 has been output, the head carries out sampling of crosstalk components of the pilot signal f1 on the track B of the third step. A signal obtained by extracting the crosstalk components of the pilot signal f1 which has been subjected to sampling by the sampling pulse SP2 from the reproduced signal having been previously subjected to sampling becomes an ATF error signal. Such a sense operation is carried out by the ATF block 3 shown in FIG. 5 which will be described later.
When the PCM sound data is reproduced, the above-mentioned ATF signals and subcodes are used.
There are two kinds of subcodes (subsignals). One is a control signal required for reproducing PCM sound data such as a sampling frequency, or the number of channels, etc. The other is a sub-channel signal for introducing music number, time or image signal attendant thereto. The former subcode is called ID (Identification Code). Particularly, the subcode recorded on a PCM sound data area is called PCM-ID and the subcode recorded on the subcode area is called subcode ID. Since signals recorded on the subcode area (subcode, subcode data, subcode ID, or control ID, etc.) can be subjected to after-recording without being erased irrespective of the PCM sound data, they are utilized for recording a program number of a time code, etc.
FIG. 8 is a schematic view showing one track pattern on a magnetic tape.
As shown in this figure, the track is composed of, from the left toward the right in the figure, in order to running of the head, a subcode area SUB1, an ATF signal area ATF1, a PCM sound data area PCM, an ATF signal area ATF2, and a subcode area SUB2. The subcode areas SUB1 and SUB2 comprise a subcode, a subcode data, a subcode ID, and a control ID, etc. Graphic data requiring large capacity, etc. are recorded on the subcode data. In addition, time code, etc. are recorded on the subcode ID because only a small data capacity is required.
The control ID is composed of a TOC-ID which is a signal indicating the presence or absence of a TOC (Table of contents), a shortening ID which is a signal indicating a fast feed to a next start ID if this represents "1", a start ID (S-ID) which is a signal indicating the start of the music and the division of music, and a priority ID which is a signal indicating the presence or absence of after-recording of the music. Particularly, since S-ID is a signal indicating the head of the program, it is a useful signal among various kinds of music signals peculiar to DAT. This signal is recorded from the head music, e.g., for nine seconds (standard mode). At the time of reproduction, this signal is searched to detect the head position of music.
Meanwhile, there are at least two kinds of modes for recording and reproducing the PCM sound data. One is a standard mode (first mode) having a sampling frequency of 48 KHz, two channels, and a linear quantization of 16 bits. The other is a non-linear long time mode (or a half-speed mode, second mode) having a sampling frequency of 32 KHz, two channels, and a non-linear quantization of 12 bits. Actually, there are also a mode having a sampling frequency of 44.1 KHz, and a mode having a sampling frequency of 32 KHz, four channels, and a non-linear quantization of 12 bits, etc. Such modes have the same recording/reproducing time as that of the standard mode.
In the half-speed mode, a revolving speed of a rotary drum and a tape running speed are set to values one-half of those in the standard mode, respectively, and a digital signal (precisely speaking, ATF signals and clock pulses for generating PCM sound data) is set to have a frequency one-half of that in the standard mode. Thus, the operating speed of the entirety of the apparatus becomes equal to one-half of that in the standard mode. Accordingly, in the half-speed mode, by allowing the operation speed of the entirety of the apparatus to be one-half of that in the standard mode although sound quality is somewhat degraded as compared to that in the standard mode, it is possible to conduct a recording/reproducing for a time twice longer than that in the standard mode with respect to a magnetic tape of the same length.
The speed for carrying out a fast forward (FF), or a rewinding (REW) of a track recorded by the abovementioned respective modes is, e.g. 200 times larger than that at the time of a regular speed running in the standard or half-speed mode. For making a high speed search at this speed, it is required to read the program number, the time, and the start ID, etc.
FIG. 9 is a schematic view for explaining that subcodes are subjected to after-recording on tracks; FIG. 10 is a schematic view for explaining that a signal subject to recording/reproducing is distorted by a recording/reproducing mode switching signal; FIG. 11 is a schematic view for explaining an offset between the track center and the locus defined by scanning of the head; and FIG. 12 is a diagrammatical view showing the arrangement in which a reproducing amplifier and a recording amplifier are subjected to switching control by a recording/reproducing mode switching signal.
As shown in FIG. 9, when a subcode on a track is subjected to after-recording, ATF1 and ATF2 signals obtained from heads having different azimuth angles were used to conduct a running position control of the heads so that the heads running on the track run in the center thereof (shown in FIG. 11). However, since the time interval from the reproduction of ATF1 to the reproduction of ATF2 is different from the time interval from the reproduction of ATF2 to the reproduction of ATF1, an offset occurred between the center of the locus defined by scanning of the head for after-recording and the center of the track as indicated by slanting lines in FIG. 9, resulting in a requirement for offset adjustment.
This will be described in greater detail with reference to FIG. 11 wherein a solid line represents the center of the locus defined by scanning of the head and broken lines represent the center of the track. An offset value from the center of the track to the center of the locus defined by scanning of the head is expressed as follows. When a distance from ATF1 to the center of the locus defined by scanning of the head is designated by a, a distance from ATF2 to the center of the locus defined by scanning of the head is designated by b, an inclination of head scanning with respect to this track is designated by e, an offset is designated by .epsilon., and letting EQU (Time interval controlled by ATF1):(Time interval controlled by ATF2)=1:r
the following relationships hold: EQU a:b=r:1, and EQU a+b=e.
From these relationships, EQU a={r/(1+r)}e, and EQU b=e/(1+r).
In an ideal tracking condition, a=b=1/2e.
In addition, an offset .epsilon. is expressed as follows: ##EQU1##
For example, when the curvature e of the track is 5 .mu.m, the offset .epsilon. becomes equal to 0.7 .mu.m (r=1.78).
As stated above, during the scanning of the head, ATF1 and ATF2 in the ATF signal area of each track were picked up, thus to conduct a tracking control using these signals.
Further, as shown in FIG. 8, the head running on the track reproduces a signal in the ATF signal area ATF1 after reproduction of the subcode in the subcode area SUB1 existing at the initial end of the track. As shown in FIG. 12, a signal to be reproduced from the head 1a (1b) is to be reproduced by the reproducing amplifier 2a through a changeover switch. In this instance, however, the input impedance of the reproducing amplifier 2a is high.
For this reason, immediately after the operation of the head running on the track is switched from recording to reproducing mode, as shown in FIG. 10, a signal to be reproduced is greatly distorted at this switching portion (immediately after the operation is switched from recording to reproducing mode), thus rendering it impossible to reproduce. Since ATF1 exists at this switching portion, this signal is not reproduced. As a result, a head running position control signal for eliminating a tracking error could not be generated from ATF1 and ATF2 on each track, and disturbance of the tracking error occurred, so that an after-recording of the subcode could not be conducted well.
In accordance with the above-mentioned rotary head type digital signal recording/reproducing system, the tracking control of the head is conducted using ATF1 and ATF2 obtained as a result of the fact that heads having different azimuth angles carry out one scanning at the time after recording of the subcode, respectively. Thus, since the intervals between ATF1 and ATF2 obtained from the heads having different azimuth angles are different, an offset occurred between the head running and the track, resulting in the requirement of offset adjustment.
In addition, when switching of the recording or reproducing amplifier selectively connected to the head is conducted at the time of after-recording of the subcode, particularly at the time of switching from recording to reproducing mode, a signal subject to recording/reproducing was distorted. As a result, the reproduction of ATF1 used for tracking control becomes impossible, so that a good tracking control cannot be conducted. Thus, there was the possibility that the subcode could not be excellently subjected to after-recording in the subcode area.